Flexible, combustion-modified polyurethane foams are known. See, e.g., U.S. Pat. Nos. 4,381,351 and 4,546,117. It is also known to produce such foams by reacting isocyanates with polyether polyols in the presence of melamine and, optionally, in the presence of various phosphorous flame retardants. See, U.S. Pat. Nos. 4,644,015, 4,745,133, 4,757,093, 4,757,094, 4,803,229, and 4,826,884, and British Patents 2,163,762 and 2,177,405.
U.S. Pat. No. 4,258,141 describes the production of flexible, combustion-modified polyurethane foams by reacting polyether polyols with a polymethylenepoly(phenyl isocyanate) containing from 40 to 90% by weight of methylene-bis(phenyl isocyanate), in the presence of melamine.
U.S. Pat. Nos. 4,822,517 and 4,900,760 describe the production of flexible polyurethane foams using a specific polyisocyanate mixture which comprises: (a) from 73 to 77 percent by weight of methylene-bis(phenyl isocyanate), having an isomer distribution of from 30 to 34% by weight of the 2,4'isomer, from 60 to 66% by weight of the 4,4'-isomer and from 3 to 7% by weight of the 2,2'-isomer, and (b) from 23 to 27% by weight of higher functional methylene-bridged polyphenyl polyisocyanates. The foams described in these references did not contain flame retardants.
While many of the flexible combustion-modified polyurethane foams known in the art have met with some commercial success, such foams are generally deficient in one or more properties and are thus not useful for many applications. It was an object of the present invention to produce a foam having (i) excellent flame properties as measured by the Oxygen Index Test (ASTM D2863) and the Radiant Panel Test (ASTM 162), (ii) good tear strength as measured by ASTM Tests D3574 and D1938, (iii) good compression set and particularly good 50% set, and (iv) good softness properties.